1. Field of the Invention
The present invention relates to a screen display circuit apparatus for displaying a dot pattern of characters, symbols, etc. on the television (TV) screen.
2. Description of Related Art
FIG. 1 is a block diagram showing tile configuration of a conventional screen display circuit. In FIG. 1, numeral 1 indicates a CPU, 2 a RAM in which a data of character disposition for display is written, 3 a ROM for storing a system program and a screen display program, 4 a character RAM in which data for determining the outputting order of character data is written, 5 a character ROM for storing font data of a plurality of characters, 9 a position control circuit for controlling the outputting timing of the font data from the character ROM 5 in accordance with the screen display position of the display pattern generated by the character RAM 4 and the character ROM 5, 6 a parallel/serial conversion circuit (hereinafter referred to as the "P/S conversion circuit") for converting the parallel output of the character ROM 5 into serial data, and 7 an RGB output terminal connected with an RGB terminal of the TV cathode ray tube not shown.
Now, the operation will be explained.
In accordance with the system program of the ROM 3, the CPU 1 is driven to write the screen display program in the RAM 2. The CPU 1 transfers the data for determining the displaying order of characters in accordance with an external signal from the RAM 2 to the character RAM 4. Assuming that the font data [A] is stored at an address 1, [B] at an address 2, [C] at an address 3 and [D] at an address 4 of the character ROM 5, for example, when the data to display characters in the order of the addresses 1 to 3 to 2 to 4 is transferred to the character RAM 4, the character ROM 5 outputs the parallel data of the characters in the order from A to C to B to D to the P/S conversion circuit 6. The outputting timing at this time is controlled by the position control circuit 9 in accordance with the screen display position of the display pattern. The information on the screen display position is input to the position control circuit 9 from the ROM 3 in accordance with an external signal.
The P/S conversion circuit 6 converts the parallel output from the character ROM 5 into serial data. The serial data "ACBD" is outputted from the RGB output terminal 7, the information "ACBD" is inputted from the RGB terminal, and the character "ACBD" is accordingly displayed on the TV screen.
The channel number, sound volume, menu, etc. have so far been commonly displayed on the TV screen. In recent years, a variety of displays has been increasingly demanded to allow easy-to-see display including a multiwindow display as high in quality as a personal computer.
The conventional screen display circuit described above, however, cannot meet the aforementioned demand. For example, it is impossible in the conventional circuit to shift or superpose the characters, symbols, etc. on the screen during the display.